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April 13, 2018 by Matt Hanson

Combining DOAS and VRF as an HVAC Solution

DOAS and VRF

For decades, using Variable-air-volume (VAV) systems with air terminal units developed extensively in commercial/institutional buildings in the United States. Unfortunately, the optimized design of a VAV system with terminal heat is difficult at best. Mostly because of limitations inherent in VAV and complications posed by design standards and regulations. One new approach involves the pairing of a dedicated outdoor-air system (DOAS) with a variable-refrigerant-flow (VRF) system. By separating the goal of achieving ventilation rates from that of maximizing thermal comfort, we avoid conflicting situations forcing compromises. What’s more, we can simplify the design process and find system efficiencies that go far beyond those commonly achieved with VAV systems with terminal heating.

In the simplest VAV system, mixing incoming outside air and return air in a central air-handling unit (AHU) and then pre-heated or pre-cooled occurs. For each zone, a terminal unit adjusts airflow based on cooling demand. When a zone requires heating, reducing supply-air flow to a minimum setting and heated, typically via a terminal-unit heating coil is the solution.

In its simplest form, a DOAS is an AHU dedicated to ventilation, not sized to provide cooling air. DOAS often are supply-only systems with relief to outdoors; however, they also can include exhaust heat recovery. Generally, sizing does not provide 100 percent air economization (cooling using outside air in lieu of mechanical cooling).

VRF systems use individual high-efficiency fan coils in interior spaces in combination with high-efficiency condensing units that serve multiple zones. The arrangement of VRF systems provides energy recovery, moving heat from zones requiring constant cooling to zones that sometimes require heating.

Meeting Industry Standards

ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, provides minimum outdoor-air-flow requirements for design conditions. ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except for Low-Rise Residential Buildings. Meanwhile, this requires some systems operations so ventilation capacity modulates to match ventilation load (i.e., demand). Designing systems to meet both of these requirements is complex.

To help match capacity to load, ANSI/ASHRAE Standard 62.1 allows “dynamic reset,” but leaves the details to the designer. A common approach combines system-level outside-air-damper reset with zone-level demand-control-ventilation (DCV) strategies. For VAV-system energy efficiency maximization, the system-level intake/exhaust/relief-damper-reset sequence is set for space-temperature control.  This aide in the coordination of building pressure with an air-economization-reset sequence. Such controls can are quite complex, particularly when considering pressurization between zones and interaction with exhaust systems requiring variable-volume control. Weather conditions also can have a major impact on building pressurization and space ventilation.

Other common control sequences for large VAV systems include occupied-hours control, optimal start/stop, fan-pressure reset. It can also include (“critical terminal unit” control), “ventilation optimization”1 (DCV), ventilation space-temperature setback, supply-air-temperature reset. Finally, included are dynamic space-pressure control, economizer, energy recovery, natural ventilation, and interfaces with building lighting controls, smoke detection, fire alarms, and even security systems. The interplay between these control schemes vastly complicates efforts to optimize building energy use.

Meeting Zone Requirements

Further complicating VAV-system design is the need to follow the ANSI/ASHRAE Standard 62.1 ventilation-rate procedure. This standard asks designers to use the multiple-space equation (MSE) to calculate the “critical” zone, the space driving the overall (system-level) outside-air fraction. Zone-level flow changing follows to meet zone-level requirements. Once the critical zone has been chosen, heating-turndown requirements push designers to over-ventilate some zones by increasing heating minimum settings. This typically changes the critical zone and reduces the overall outside-air fraction at the central-system level, substantially affecting AHU components.

To respond to changes in zone population, dynamic reset of VAV systems, they often apply combining zone-level DCV with system-level ventilation reset. Ventilation reset is a control scheme by which the MSE is solved dynamically to change the system outside-air setpoint. This application with airflow-measuring stations, along with electronics and software to control dampers based on relative airflow, happens at the central system.

Maximizing Energy Efficiency

Central-system design usually is based on the static-condition (design) critical zone. While in the real world, the zone population varies dynamically along with building HVAC load (internal and weather-related) over the course of a day. Maximizing energy efficiency under these conditions pushes the envelope in terms of building system design and building-operation hardware and software. Some commercial software packages can aid ventilation reset, calculating critical ventilation zone per ANSI/ASHRAE Standard 62.1. Calculating the minimum system outside air, however, usually requires the overriding of some terminal-unit turndowns, which changes the critical zone.

This is allowed by ANSI/ASHRAE/IES Standard 90.1 because only a few zone overrides can change a system outside-air setting by many percentage points. This saves considerable pre-treatment energy and reducing central-system size. Meanwhile, inputs to these calculations vary with system load, which also can change the critical zone. At the same time, other overrides, such as of supply airflow for makeup to areas with exhaust flows exceeding ASHRAE minimum rates, are necessary. These overrides and simultaneous variations result in the need for iterative calculations involving all environmental variables; such calculations currently are beyond the scope of commercially available software.

Zoning is another limitation as history shows, building-construction economics can drive VAV-system designers to combine up to several rooms on a single ventilation and temperature-control zone. Because heating and cooling loads can vary widely between rooms in a single zone. This often leads to discomfort in certain rooms when other rooms in the same zone are not at the design load.

To Sum it Up DOAS and VRF

In summary, designing a VAV multi-zone HVAC system can be challenging to for DOAS and VRF, to say the least. Overlaying the various requirements, exceptions, and system functions results in iterative design simulations that must be re-run whenever a room’s size changes. Complying with all codes while addressing the competing interests of ventilation and energy optimization is theoretically possible. However, technical solutions partly exist and are being developed. In practice, designing and redesigning systems with complex, iterative calculations is not very practical, and manual overrides generally do not fully optimize system designs. Meanwhile, designers often estimate “block” (net) load based on building-envelope heating and cooling. Considering neither variations in internal and ventilation heating/cooling loads nor airflows needed for ventilation and space pressure control, significantly.  The result is undersized or oversizing systems.

Fortunately, a design paradigm is emerging to compete with the old VAV model. This new model can reward us with simplified system design while letting us achieve the increased system efficiencies that energy costs are demanding

Filed Under: News Tagged With: #hvac #hvaclife #plumbing #hvactechnician #airconditioning #heating #hvactech, DOAS, VRF

April 10, 2018 by Matt Hanson

Grow Room HVAC Units

The rapid increase of the cannabis industry within the United States creates a critical need for proper HVAC equipment in Grow Room facilities. Growing cannabis can be very tricky due to the fact it is grown indoors instead of outdoors like other crops. It is critical that cooling and dehumidification are the focus of their HVAC needs.
There are six main challenges that one will face when determining an HVAC system for a grow facility. Those six challenges are temperature, humidity, lighting, airflow, ventilation, and smart technology.

Grow room facilities typically need to be between 70°F-75°F with humidity between 50-60%. HVAC systems should be able to maintain the specific temperature to ensure growth for the plants.  Also needed is the ability to quickly remove any additional humidity that may arise during the watering process. The lights used in these facilities also produce a large amount of heat, therefore, needing a strong HVAC unit to keep the rooms cool and prevent overheating.

Airflow is critical and your HVAC system should provide enough to create a “natural like breeze” to ensure the plants have constant movement and vibration. Equally important is the need to provide extensive filtration to the system. This will ensure smells cannot leak to the outside and prevents mold spore formation by capturing all dust and sediment. Lastly, your grow room cooling unit must have smart technology enabled in order to control all of the above mentioned or alert you if something goes wrong.


When it comes to selecting your HVAC unit for a grow room facility, you need to think of United CoolAir. United CoolAir can manufacture exactly what you are looking for in order to create a successful grow room. We have both the expertise and experience to provide you the best operational equipment in the USA.

Filed Under: News Tagged With: #growroom, #hvac #hvaclife #plumbing #hvactechnician #airconditioning #heating #hvactech

December 13, 2017 by Matt Hanson

You’ll Call it Remarkable, We Call it EZ-FIT

EZ-Fit modular floor-by-floor unit

The Variable Air Volume (VAV) VariCool® EZ-Fit unit’s specific design is for the cost-effective replacement of existing floor-by-floor or constant volume purposes. These self-contained floor-mounted systems are modular in design and consist of three or four sections. Single module sizes range in capacity from 12 – 45 tons. Individual combined modules are able to achieve 90 tons of total cooling capacity with either air or water-cooled configurations.


The EZ-Fit design is especially important in multi-floor structures with limited space and accessibility. It’s an All-Indoor Solution specifically for installation in renovation and retrofit applications.
When compared to traditional air conditioners, United CoolAir has the Lowest Installed Cost! Each of the VariCool® EZ-FIT sections is capable of fitting through a standard 3-0 doorway. The units easily negotiate corners, narrow hallways and can be placed into service elevators. The result is no building modification, fast access to the mechanical room, and no unnecessary handling, rigging, or permits.

Installations completed over the Weekend


The installation of modules happens over a weekend or during plant shutdown, minimizing disruption in services or workflow. United CoolAir custom manufactures all its equipment to the customer’s unique requirements. Utility and duct connection match the original locations eliminating costly modifications.


UCA works closely with the installer to deliver systems that coincide with the retrofi t schedule. Preplanning eliminates space issues at the job site, and system components go from truck to installation site saving valuable time and manpower. UCA’s Marvel Plus microprocessor controller is compatible with all major Building Management Systems (BMS).
To ensure fast, flawless installation, floor-by-floor factory-assembled modules, evacuation, charging, and testing occur before shipment. The disassembled units are split into three or 4 sections before shipment. Sections assemble easily and include re-sealable refrigerant fittings that maintain the factory charge.


The All Indoor installation feature has other benefits as well! To preserve the architectural integrity of the structure, all components are inside the building. No field testing is necessary since the entire system is factory tested before shipment. The VariCool EZ-FIT offers protection from the weather and vandalism extending its usable life. Furthermore, customers can achieve lower costs because routine outdoor servicing and maintenance are eliminated.

Filed Under: News Tagged With: #hvac #hvaclife #plumbing #hvactechnician #airconditioning #heating #hvactech, #UCA

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